The field of the invention is systems and methods for medical imaging using vibratory energy, such as ultrasound imaging. More particularly, the field of the invention is systems and methods for shear wave dispersion ultrasound vibrometry (“SDUV”).
Tissue mechanical properties are linked to tissue pathology state. Shear wave propagation methods have been proposed to quantify tissue mechanical properties. In these methods, shear waves that result from a transient (impulsive or short tone burst) excitation of tissue propagate only a few millimeters, as a result of tissue absorption and shear wave attenuation, therefore boundary condition problems are overcome, allowing us to assume that the shear waves propagate as if in an infinite medium. Shear waves are usually generated by external mechanical vibration or by acoustic radiation force from a focused ultrasound beam. The advantage of using acoustic radiation force is that if an acoustic window is available then the ultrasound system can create a focused beam to apply radiation force to push tissue.
Characterization of tissue mechanical properties, particularly the elasticity or tactile hardness of tissue, has important medical applications because these properties are closely linked to tissue pathology state. Recently, shear wave propagation methods have been proposed to quantify tissue mechanical properties. In general, these methods generate shear waves in tissue by transiently exciting the tissue. These shear wave propagate a short distance, such as only a few millimeters, because of tissue absorption and shear wave attenuation. Shear waves are usually generated by external mechanical vibration or by acoustic radiation force from a focused ultrasound beam. The advantage of using acoustic radiation force is the fact that a pushing pulse of radiation force can be applied anywhere an acoustic window is available so that an ultrasound system can create a focused ultrasound beam.
One example of an ultrasound technique for measuring mechanical properties of tissues, such as elasticity and viscosity, is called shear wave dispersion ultrasound vibrometry (“SDUV”). This SDUV technique is described, for example, in U.S. Pat. Nos. 7,753,847 and 7,785,259, which are herein incorporated by reference in their entirety. In SDUV, a focused ultrasound beam that operates within FDA regulatory limits, is applied to a subject to generate harmonic shear waves in a tissue of interest. The propagation speed of the induced shear wave is frequency dependent, or “dispersive,” and relates to the mechanical properties of the tissue of interest. The motion of the tissue is measured using pulse-echo ultrasound techniques. Shear wave speeds at a number of frequencies are measured and subsequently fit with a theoretical dispersion model to inversely solve for tissue elasticity and viscosity. These shear wave speeds are estimated from the phase of tissue vibration that is detected between two or more points with known distance along the shear wave propagation path.
For a viscoelastic, homogenous, isotropic material, the shear wave speed, cs, and shear wave attenuation, αs, are related to the complex shear modulus, G*(ω)=Gs(ω)+iGl(ω), by:
                                                        c              s                        ⁡                          (              ω              )                                =                                                    2                ⁢                                  (                                                                                    G                        s                        2                                            ⁡                                              (                        ω                        )                                                              +                                                                  G                        l                        2                                            ⁡                                              (                        ω                        )                                                                              )                                                            ρ                ⁡                                  (                                                                                    G                        s                                            ⁡                                              (                        ω                        )                                                              +                                                                                                                        G                            s                            2                                                    ⁡                                                      (                            ω                            )                                                                          +                                                                              G                            l                            2                                                    ⁡                                                      (                            ω                            )                                                                                                                                )                                                                    ;                            (        1        )                                                                    α              s                        ⁡                          (              ω              )                                =                                                                      ρω                  2                                ⁡                                  (                                                                                                                                          G                            s                            2                                                    ⁡                                                      (                            ω                            )                                                                          +                                                                              G                            l                            2                                                    ⁡                                                      (                            ω                            )                                                                                                                -                                                                  G                        s                                            ⁡                                              (                        ω                        )                                                                              )                                                            2                ⁢                                  (                                                                                    G                        s                        2                                            ⁡                                              (                        ω                        )                                                              +                                                                  G                        l                        2                                            ⁡                                              (                        ω                        )                                                                              )                                                                    ;                            (        2        )            
where ρ is the density of the material; ω is the angular frequency of the shear wave; Gs(ω) is the storage or elastic modulus; and Gl(ω) is the loss or viscous modulus. Quantitative mechanical properties can be measured in a model independent manner if both shear wave speed and attenuation are known; however, measuring shear wave attenuation is challenging in the field of elasticity imaging. Typically, only shear wave speed is measured, and rheological models, such as Kelvin-Voigt, Maxwell, and Standard Linear Solid, are used to solve for complex shear modulus.
Acoustic radiation force has been used to study quasi-static viscoelastic properties of tissue. Transient characteristics of viscoelastic materials are known as creep and stress relaxation. Creep is a slow, progressive deformation of a material under constant stress. Stress relaxation is the gradual decrease of stress of a material under constant strain. Tissue creep response to an applied step-force by means of acoustic radiation force has been shown in several studies. In one such study, Mauldin, et al., reported a method for estimating tissue viscoelastic properties by monitoring the steady-state excitation and recovery of tissues using acoustic radiation force imaging and shear wave elasticity imaging. This method, called monitored steady-state excitation and recovery (“MSSER”) imaging, described in U.S. Patent Application No. 2010/0138163, is an noninvasive radiation force-based method that estimates viscoelastic parameters by fitting rheological models, Kelvin-Voigt and Standard Liner Solid model, to the experimental creep strain response. However, as in shear wave propagation methods, a rheological model needs to be fit to the MSSER experimental data to solve for viscoelastic parameters.
Current elasticity imaging techniques are useful to identify tissue mechanical properties; however, to quantify these properties a rheological model must be used, which introduces an undesirable amount of computational burden to the quantification process. In additional, rheological models may not describe the material behavior at all frequencies, may not be appropriate for the physical test being performed, and are less general than a model-free approach.
Acoustic radiation force induced creep can be used to quantify viscoelastic parameters in a model-independent way by estimating the complex shear modulus from creep displacement response. In this method, high-intensity pushing beams are interspersed with conventional-intensity tracking beams, both being applied during the creep period. With this approach, both creep and recovery response are induced, but only creep displacements are used to estimate the model-independent complex shear modulus. A limitation of this current method, is that to study viscoelastic parameters in a wider range of frequencies, the creep period needs to be maintained for long times, for instance up to 30 ms, which means that the total exposure time to acoustic radiation force will increase.
It would therefore be desirable to provide a system and method for estimating viscoelastic parameters in a model-independent manner while not exceeding FDA limits on mechanical index (“MI”), which may otherwise be violated by current approaches to generating and monitoring creep response.